Which of the following is true of environmental changes?
a. they cannot be caused by human activity.
b. they may affect several populations in an ecosystem.
c. they affect food chains, but not food webs.
d. they have no effect on living organisms in an ecosystem.

The world wars of the twentieth century immediately led to a. the establishment of a communist bloc in eastern europe b. the urbanization of europe’s societies c. an emphasis on humanism d. an emphasis on ties to greek and roman societies select the best answer from the choices provided. a b c d